#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv) 
{
	int id, N, np, i;
	double sum, left;
	if (argc != 4) 
	{
  		printf("Uso:\n%s id N np\n",argv[0]);
		exit(1);
	}
	id = atoi(argv[1]);
	N = atoi(argv[2]);
	np = atoi(argv[3]);
	for(i=id, sum = 0; i<N; i+=np) 
	{
		double x = (i + 0.5)/N;
		sum += 4 / (1 + x*x);
	}
	sum /= N;
	printf("%lf\n", sum);
	return 0;
}
